1. Technical Field
The present invention relates to an intrusion detector with power consumption control. More specifically, the invention relates to a vehicle intrusion detector of the type based on Doppler effects of transmitted ultrasonic waves. The invention also relates to a method for intrusion detection.
2. Background Information
Intrusion detectors are known for creating unwanted false alarms. This is particularly true in vehicle intrusion detectors. Not only do they create a disturbance to people in the neighborhood and risk reducing or eliminating the power source, they also tend to result in common acceptance of active alarms, thereby decreasing the discouraging effect of alarms on potential burglars.
Detectors based on transmission of waves in, for example, the interior of a car have been developed and are known in the art. Typically, these systems receive waves reflected from the interior surroundings, demodulate them, pass them through a band pass filter, integrate the band pass signal, and compare the result with a predefined threshold value. If the threshold is exceeded, a signal is passed to a unit responsible for giving off an alarm. The waves are often ultrasonic sound waves or microwaves, and the demodulation performed is often based on the Doppler effect.
U.S. Pat. No. 5,856,778 to Kani et al. discloses a vehicle intrusion detector of this type that is an improvement of this wave transmission technique. The detector of Kani also includes an envelope detecting circuit for determining the level of the received ultrasonic signal. An alarm is generated only if this ultrasonic level exceeds a second predefined threshold value.
Intrusion alarms also face the problem of power consumption, especially in a vehicle or any other location where power is a scarce resource. A vehicle alarm is active only when the vehicle is parked and turned off, making it completely dependent upon the available battery power. If the vehicle is parked during a long period, it is crucial that power consumption is minimized, as the vehicle otherwise will be difficult to start.
In a detector such as the one described in U.S. Pat. No. 5,856,778, the issue of power consumption differs with the first issue of avoiding unwanted false alarms, as alarm accuracy depends on a relatively strong ultrasonic wave transmitter, and therefore increased power consumption. In order to overcome excessive power consumption, the supersonic transmitter is driven intermittently. However, this approach is not free from problems. First, the ability of the detector to detect any intrusion is greatly reduced, as it is active only part of the time. During inactive or intermittent periods, the detector is xe2x80x9cblindxe2x80x9d, allowing intrusions, although short, to occur unnoticed. Secondly, the output frequency of the supersonic transmitter is unstabilized at the beginning of each active period, making it difficult to stabilize the detection operation of the detector. Thus, there is an increased risk of obtaining a false alarm due to this unstable period occurring in each active period.
A solution for this problem is provided in U.S. Pat. No. 5,483,219 to Aoki et al. Aoki discloses a device that invalidates the signal from the integrating circuit during a short period of each active period. Still, this is not a completely satisfactory solution. Further, it does not in any way improve the partial xe2x80x9cblindnessxe2x80x9d of the detector.
As such there is a need for an intrusion detector that is able to combine a high detection level, good signal quality, and low power consumption.
The present invention provides an intrusion detector that overcomes the above problems, making it possible to continuously detect intrusions while simultaneously reducing power consumption. The invention also provides a vehicle intrusion detector with continuous transmission of waves having satisfactorily low power consumption.
These and other objects are achieved by a detector having a transmitter, receiver, detector, and control unit. The transmitter has a variable output power and is able to transmit waves into a defined space. The receiver is able to receive those waves after they reflect. The detector is connected to the receiver and is able to detect a signal based on a change in the waves after reflection, with the detected signal being representative of any motion in said defined space. The control unit is able to determine any change in the detected signal, as well as increase the output power of the transmitter in response to such change. They are also achieved by a method for intrusion detection involving transmitting waves into a defined space, receiving those waves after reflection, detecting a signal based on a change in the waves after reflection, where the reflected signal is representative of any motion in the defined space, controlling an output power of the wave transmission, and increasing the output power in response to a change in the detected signal.
When using a detector according to the invention, waves are transmitted into a defined space, and reflected back to the receiver (sensor), and the reflected waves are processed to obtain a signal corresponding to any change caused by motion in the space. The output power of the transmitter is variable, and a micro-controller (MCU) is arranged to increase the output power in response to a change in said signal.
This arrangement makes it possible to avoid driving the transmitter with a high power output, unless a signal representing motion in the vehicle has been detected. This in turn allows the power consumption to be lowered considerably without losing the continuous transmission of waves into the space.
According to a preferred embodiment, the detector includes a signal processor that is able to process the detected signal in order to securely determine if an intrusion has occurred. This processing can be of any suitable type, possibly of a previously known kind. It is typically dependent on a satisfactory signal-to-noise ratio in order to perform the analysis.
During normal functioning when no motion is detected, the transmitter output power is kept at a low level. A change in the reflected waves thus results in a very weak signal that cannot be securely analyzed in order to eliminate false alarms. As soon as the signal is detected, implying that any motion is present in the vehicle, and the transmitter output increased by the MCU, a significantly stronger detected signal is provided, considerably increasing the signal-to-noise ratio. This makes it possible to use advanced detection algorithms whereby the detected signal can be analyzed in order to securely determine whether or not the detected change is due to an intrusion. In the case of extreme weather conditions or traffic noise that result in high signal levels, those detected signals can be identified and removed by advanced algorithms without any significant increase in power consumption.
To further decrease power consumption, the signal processor can be deactivated during periods of low power transmission. The control unit is then arranged to activate the processor only when a change in the detected signal is determined, i.e., when the output power is increased.
One preferred way of embodying the output power control is to allow the control unit to raise the output power of the transmitter to a higher output level when the detected signal exceeds a predefined threshold.
Preferably, the control unit is then arranged to reset the output power of the transmitter to a lower output level after a predetermined period of time without any intrusion detection. Each time the detected signal exceeds the threshold, the intrusion detector will then be placed in an alert condition with a higher output power and a significantly increased possibility of identifying intrusions. If the detector signal increase is found to be a xe2x80x9cfalsexe2x80x9d alarm and no intrusion can be securely determined, the output power will be reset again, thereby effectively reducing power consumption.
The waves that are transmitted into the defined space can be any type of waves that can be carried by the medium filling the space. In the normal case of an air-filled space, possible wave types include sound waves and electromagnetic waves (e.g., microwaves or infrared waves).
In one embodiment of the invention primarily intended for use in a vehicle, the waves are ultrasonic waves. These waves have the advantage of being evenly distributed in the vehicle, while at the same time being confined by the vehicle walls and windows. Also, at least compared to microwaves, ultrasonic waves have a lower frequency, facilitating detection of any change in the reflected waves caused by an intrusion.
With microwaves, the transmitter generates a well-defined wave field, making good detection possible in defined areas. An advantage with using microwaves is that the detector is undisturbed by air movements, for example, movements due to an open window. On the other hand, the wave field must be carefully adjusted within the space. Otherwise, movements outside a window can be unintentionally detected.
As mentioned, infrared waves are another possible wave form. However, infrared waves are more sensitive to temperature changes and light phenomena typically introduced through the windows of a vehicle.
The signal detected by the detector in the reflected waves can be of different types, as the detection can be based on various types of phenomena occurring in the reflected wave field.
In one embodiment of the invention the detection is based on the Doppler effect. This technique is commonly known in intrusion detectors, including those found in vehicles as well as in other types of movement detectors. The technique can be utilized with ultrasonic waves as well as with electromagnetic waves. An advantage of using the Doppler effect is that the geometric form of the space is less influential on the result, which is advantageous in, e.g., a vehicle with a typically complex geometry. When using the Doppler effect in connection to microwaves, the detection can be based on pulse Doppler effect.
The detection of the Doppler signal can be performed with an envelope demodulator. The envelope of the reflected waves has the advantage of being phase and frequency independent, i.e., synchronization is not crucial. This in turn results in a greater freedom when placing the detector in a convenient space, such as a vehicle passenger compartment, especially in vehicle intrusion detectors, as it allows for a more compact design.
Instead of Doppler effect, a time discriminating detection can be performed. In the case of microwaves, a combination of these techniques can be performed, for example used in the pulse Doppler radar.
The detector according to the invention is primarily intended for the interior of a vehicle, such as a personal car, van, truck, etc., but could in principle be applied to any type of alarm system where low power consumption is desired.